Electroluminescent system, electrically non-linear element and method



w. LEHM ELECTROLUMINESCENT SYSTEM, ELECTRICALLY NON-LINEAR ELEMENT AND METHOD Filed May 2, 1960 2 Sheets-Sheet 1 FIG. I. FIG. 2.

NON-LINEAR MATERIAL NON-LINEAR MATERIAL NON-LINEAR MATERIAL FIG; 3.

Ac F12 FIG.4.

BRIGHTNESS IN ARBITRARY UNITS O BY 50 I00 200 300 400 600 VOLTS W5 pm 1963 w. LEHMANN ,075,122

ELECTROLUMINESCENT SYSTEM, ELECTRICALLY NON-LINEAR ELEMENT AND METHOD Filed May 2, 1960 2 Sheets-Sheet 2 53 FIG. 5. I

NON- LINEAR MATERIAL 44 46 46 SIGNAL GENERATOR A I I I l l l 54 48 j If/, NON-LINEAR MATERIAL 6O INVENTOR. W/A l L E M/i/V/V FIG. 7.

nite tt This invention relates to electroluminescent systems and, more particularly, to an electroluminescent system displaying a brightness which increases rapidly with increasing alternating potential excitation, an electrically non-linear element and methods for preparing an electrically non-linear material.

For some applications it is desirable that the brightness of an electroluminescent device increases very rapidly with increasing potential excitation. As an example, a socalled XY electroluminescent display plotter is disclosed in Bramley and Rosenthal article, Review of Scientific Instruments, vol. 24, page 471 (1953). Briefly, such a device comprises two sets of parallel conductive strips which are oriented at an angle of 90 with respect to one another, with a layer of electroluminescent phosphor material sandwiched therebetween. When preselected individual conductors which comprise the parallel conductive strips have an energizing potential applied thereacross, the electroluminescent phosphor therebetween is excited to visible light emission. Such a system has an inherent disadvantage in that while the desired full voltage is applied between the preselected energized strips to energize a preselected area to light emission, half of this voltage is also applied between each energized strip and the remaining unenergized strips which are disposed perpendicular thereto. This results in energization of unwanted areas of the device to a lower level of illumination and this is known in the art as cross-talk.

The foregoing problem has been recognized and is thoroughly discussed in US Patent No. 2,877,371, dated March 10, 1959, wherein an additional separate layer of ferroelectric material such as barium titanate is specifically described as positioned between the cross-grid electrodes, along with the layer of electroluminescent phosphor. When the ferroelectric material is subjected to a D.C. current therethrough, the conductivity increases considerably to allow an alternating potential of increased magnitude to be applied across the electroluminescent phosphor, thereby causing the electroluminescent brightmess to increase very rapidly with a relatively small increase in potential excitation. Such a system minimizes endencies for cross-talk.

To obtain high brightness with electroluminescent devices, the layer comprising the phosphor should display a very high resistance to direct current flow and high electrical breakdown characteristics. This normally requires embedding the phosphor in a substantial amount of dielectric material or including a separate layer of dielectric material between the device electrodes, or both, so that the layer comprising the phosphor will display a very high resistance to direct current flow.

It is the general object of this invention to avoid and overcome the foregoing and other difiiculties of and objections to prior-art practices by the provision of an electroluminescent system energized by alternating electric potential, which system displays a rapidly increasing brightness with an increasing magnitude of alternating potential excitation.

It is another object to provide modifications for an electroluminescent system which displays rapidly increasing brightness with increasing magnitude of alternating potential excitation.

SfilSAZZ? Patented Jan. 22, 1963 It is a further object to provide an improved X-Y electroluminescent plotter which has substantially no tendencies for unwanted cross-talk and which plotter can be operated with very high brightness.

It is an additional object to provide an electrically non linear element which displays an electrical impedance decreasing in accordance with the magnitude of an electric potential applied thereto.

It is yet another object to provide methods for preparing a specific electrically non-linear material which displays an electrical impedance decreasing in accordance with the magnitude of an electric potential applied thereto.

The aforesaid objects of the invention, and other objects which will become apparent as the description proceeds, are achieved by providing an electroluminescent display system which is energized by alternating electric potetnial. In this system, spaced electrodes have included therebetween separate layers comprising alternating-electric-field-influenced electroluminescent phosphor and alternating-sicctric-field-infiuenced, non-photoconductive and non-electroluminescent material. When the system is operated, the electroluminescent phosphor exhibits an alternating potential drop thereacross which varies with the magnitude of the alternating potential applied across the electrodes. The non-electroluminescent material dis plays a potential drop thereacross which also varies with the magnitude of the alternating electric potential as applied across the electrodes. These two potential drops are so related that the ratio of the potential drop across the electroluminescent phosphor divided by the potential drop across the non-electroluminescent material decreases as the potential applied across the electrodes is increased. Also provided is an improved X-Y plotter which incorporates such an electroluminescent system. An improved non-linear material comprising a particular zinc oxide which can be used in such a display system and methods for making such non-linear zinc oxide are also provided.

For a better understanding of the invention, reference should be had to the accompanying drawings wherein:

FIG. 1 is a sectional elevational view of an electroluminescent system which displays a brightness which increases rapidly with increased alternating electric potential excitation;

FIG. 2 is a sectional elevational view of a system generally similar to that shown in FIG. 1, but wherein the bounding electrodes have different dimensions;

FIG. 3 is a plan view, partly broken away and partly in section, showing an alternative construction for an electroluminescent system which displays a brightness which increases rapidly with increased alternating electric potential excitation;

FIG. 4 is a graph of log. brightness versus log. applied voltage illustrating performance characteristics of an electroluminescent system constructed in accordance with the present invention;

FIG. 5 is a perspective view of an X--Y plotter constructed in accordance with the present invention;

FIG. 6 is a perspective view, partly in section, of an alternative construction for an improved X-Y plotter;

FIG. 7 is a sectional elevational view of an electrically non-linear element constructed in accordance with the present invention.

Although the principles of the present display system are broadly applicable for use with any electroluminescent application wherein the brightness is desired to increase very rapidly with applied voltage, the invention has particular utility with respect to X-Y plotters and such plotters have been so illustrated and will be so described. Also, while the non-linear element as described herein has wide utility for applications other than electroluminescence, it is particularly adapted for use with an electroillustrated in the drawings, in

3 luminescent display system and such a system has bee so illustrated and will be so described.

With specific reference to the form of the invention FIG. 1 is illustrated generally an electroluminescent system which comprises an electroluminescent device lo with an alternating electric potential source 12 connected thereto. The electroluminescent device it} generally comprises a foundation 14 which carries thereon a light-transmitting electrode layer 16. Over the electrode'layer is is a layer 13 comprising electroluminescent phosphor and a layer 2% comprising non-linear material is carried over the phosphor layer 1.8. A second electrode layer 22 is carried over the non-linear material layer 2th As aspecilic example, the light-transmitting foundation 14 is fabricated of glass and the electrode layer 16 is fabricated of tin oxide, such material being well known for this purpose. The electroluminescent layer '13 has a thickness of fifty microns and comprises any suitable electroluminescent phosphor such as zinc sulfide activated by copper and coactivated by chlorine. The phosphoris mixed in predetermined proportions with a light-transmitting dielectric material, such as equal parts by weight of phosphor and polyvinyl-chloride acetate. The electrode 22 is formed of vacuum-metallized aluminum or silver. The source 12 comprises any conventional alternating electric potential source connected direcuy to the electrodes 16 and 22 and is designed to supply potentials of different magnitudes.

The foregoing electroluminescent construction is subject to considerable variation. As an example, any suitable light-transmitting material can be substituted for the glass used in fabricating the foundation 14. The electrode in can be replaced by a mesh or grid of wires. Any suitable electroluminescent material can be used in place of the specific example given hereinbe-fore and such electroluminescent materials are well known. The thickness of the phosphor layer 13 can be varied and the relative proportions of phosphor to-dielectric can'be varied over a wide range. Other suitable dielectric materials can be used. The phosphor can also be used in thin film form with a separate layer of dielectric material. .Thesecond electrode layer 22 can be formed as a mesh of Wire or it can be made light'transmitting in nature by utilizing a tin-oxide-coated glass.

Other alternative constructions for the device as shown in FIG. 1 are disclosed in FIGS. 2 and 3. The embodiment 24 as shown in PEG. 2 is generally similar to that shown in FIG. 1 except that an additional layer 26 of dielectric material is included between the spaced electrodes,'the uppermost electrode 22a has an area which is considerably smaller than the area of the electrode layer l6 and a separate floating electrode layer 2% having an area comparable to electrode layer is is included between the non-linear layer and the additional dielectric material layer 26. The purpose of the reduced area of the electrode 22a with respect to the electrode layer 23 is to reduce the capacitive reactance therebetween, as will be explained in greater detail hereinafter.

If the bounding electrodes have equivalent areas, as in the embodiment shown in FIG. 1, the thickness of the non-linear material layer is should be at least slightly greater than the thickness of the phosphor layer 18, in order to decrease the'relative capacitive reactance of this non-linear material layer. The. relative thickness dilierentials between "the phosphor and non-linear material layers are not critical, however, provided the capacitive reactance of the non-linear layer is less than that of the phosphor layer.

in PEG. 3 is shown a further alternative embodiment wherein the device electrodes are formed as an interlacing grid mesh on a foundation 34 with a separate layer 36 comprising electroluminescent phosphor and a separate layer 38 comprising non-linear material spaced th crebetweeu. in other details, the constructions ot the embodiments as illustrated in PPS-S. 2 and 3aresimilar total gram atoms of aluminum, scandiutn,

to the embodiment shown in H6. 1. All of the foregoing electroluminescent systems thus include spacedelectrodes, an alte nating electric potential source which is capable of delivering potentials of different magnitudes connectedacross the spaced electrodes, field'influenced electroluminescent phosphor means and riclddnliuenced, non-linear means included as separate layers between the spaced electrodes. in addition, there is also included between the spaced electrodes, either in the form of mixed dielectric or in the form of a separate layer of dielectric, suiiicient insulating material to prevent any direct current flow between the spaced electrodes. This insulating material increases the brigh ness which is obtainable from the systems and in addition, the alternating electric potential which is required to cause electrical breakdown between the electrodes is increased. Tl e field-influenced, non-electroluminescent means should be non-photoconductive in nature, since a photoconductive material will require light hielding as well as some means for preventoptical feedback from the electroluminescent phosphor.

The non-linear layer 2% preferably comprises specially processed zinc oxide. 'Zinc oxide normally does not display electrically non-linear characteristics, that is, an electrical impedance which decreases when an increasing electric field is applied thereacross. A special non-linear zinc oxide can be prepared by several methods. As a first method, finely-divided zinccarbonate is fired in a covered silica crucible, which will permit egress of pressures generated therein, at a temperature of from about i000 C. to about 1200" C. for a period of from ten minutes to about six hours. The preferred tiring conditions are a firing temperature of about llllt)" C. for a period of about one hour. The resulting zinc oxide will display non-linear characteristics as described hereinafter.

As a second method, finely-divided zinc oxide is mixed with from 0.1% to 30% by weight of finely-divided sodiurn chloride. 'l'his mixture is fired in an oxygen atmosphere at a temperature of from 890 C. to 1260 C. for a period of irorn about ten minutes to about six hours. Thereafter the'fired material has separated therefrom any residual sodium chloride. Preferably, the mixed sodium chloride in the unfired mixture comprises about 10% by weight of the zinc oxide. This mixture is preferably fired in an oxygen atmosphere at a temperature of about 1000 C. for a period of about one hour. The fired mixture is thereafter Water rinsed in order to remove residual sodium chloride.

As a third and preferred method, finely-divided Zinc oxide and selected compounds are mixed the proportions of one mole zinc oxide and from about 0.0l gram atom percent to one gram atom percent of copper, lithium or silver in compound form, or mixtures thereof. Also mixed with the foregoing is from C201 gram atom percent to about one gram ato-rn-percent of aluminum, scandium, gallium or indium in compound form, or mixtures thereof. The total gram atoms of copper, lithium or silver should constitute from about one-half to twice the gallium, or indium which are present in the mixture. The foregoing mixture is tired in an oxygen-containing atmosphere a temperature of from about 1060 C. to about l3tlG C. for a period of from about ten minutes to about six hours. Any usual compounds of the copper and other indicated etallic materials can be used in preparing this zinc xide although his preferred to use the sulfate. As a preferred specific example, copper sulfate and aluminum.

sulfate in amount consistent with the foregoing range:

are'mixed with the zinc oxide and fired in an air atmos- -phere at ateruperature of from about 1160 C. to abou 1200 C. for a period of about one hour. in of th: fc'egoing examplea'the state of division of the zinc ant other metallic compounds is not critical and is subject t: considerable variation. As an example, the um'ired finely-divided zinc carbonate has an average particle siz of about 0.2 micron. The unfired zinc oxide has an average particle size of about 0.2 micron. The sodium chloride is preferably added in solution form as are the copper sulfate and aluminum sulfate or other metallic compound additions to the zinc oxide.

"The special zinc oxide as processed in accordance with any of the foregoing methods will display an electrical impedance which decreases in accordance with increasing electric field applied thereacross. In order for the zinc oxide to display non-linear characteristics which are acceptable for use in the present electroluminescent systern, the zinc oxide should be used in separate layer form and mixed with dielectric material. In addition, the particle diameter of the processed zinc oxide should be selected so that it bears a predetermined relationship with respect to the predetermined thickness of the zinc oxide di lectric layer. The non-linear layer can comprise from to 80% by weight of dielectric material and from 85% to by weight of finely-divided zinc oxide prepared as specified hereinbefore. Preferably, the dielectric material should comprise from to by weight of the layer and the zinc oxide should comprise from 70% to 60% by weight of the layer. The average particle size of the zinc oxide as prepared by any of the foregoing methods will normally vary fromv about 1 micron to about 12 microns, with the higher the firing temperature, the larger the average particle size. Even the foregoing particle size range can be extended. As a specific example, zinc oxide prepared in accordance with the specific example for the third method as given hereinbefore has an average particle size of about 5 microns. The thickness of the layer comprising the non-linear element formed by the zinc oxide and mixed dielectric material is preselecte in accordance with the average particle diameter of the zinc oxide so that such average zinc oxide particle diameter is "rom about one-fourth to about one one-hundredth of the layer thickness. Preferably, the average particle diameter of the zinc oxide is about one-tenth of the predetermined thickness of the nonlinear layer. As a further specific example, where the zinc oxide has a particle diameter of about 5 microns, 35 by weight of dielectric material and 65% by weight of finely-divided zinc oxide are mixed and formed as a layer havin a thickness of about microns and this layer is adapted to have an electrical potential applied thereacross. Any suitable dielectric material can be mixed with the zinc oxide and as a specific example, poiyvinylchloride acetate has been found to be very satisfactory.

Apparently the nonlinear characteristics of the foregoing element or layer comprising zinc oxide involve a partic e contact phenomenon between the individual zinc oxide particles when they are mixed With the dielectric material in the foregoing relative proportions. If less than the indicated amount of dielectric material is used, the resulting layer will still display non-linear characteristics, but the elect ical breakdown character istics will bc paired so that the usefulness of the element is limited. If the proportion of dielectric is greater than as indicated before, the non-linear characteristics of the formed layer are also impaired, apparently due to decreased contact points between individual zinc oxide particles. In addition, when the predetermined thickness of the layer is selected in accordance with the predetermined particle size of the zinc oxide, it has been found that there will be such contact areas between the individual zinc oxide particles as related to total impedance as are required to produce the desired non-linear characteristics for the layer or element.

In FIG. 4 are shown performance characteristic curves, expressed in log. bri htness (arbitrary units) vs. 10". applied volts, for an electroluminescent system generally as disclosed in Flu. l and wherein the non-linear layer 2% is varied in thickness from 0.08 mm. to 0.32 mm., with the thickness of the phosphor layer 18 maintained at approximately 0.05 mm. Also shown are the performance characteristics for an otherwise-similar control system which does not incorporate any separate layer comprising non-linear zinc oxide. In the curve indicated A are shown the performance characteristics for the control. The curve designated B illustrates performance characteristics for an electroluminescent system which incorporate between the spaced electrodes a separate layer comprising non-linear zinc oxide having a thickness of approximately 0.08 mm. The curves C, D and E illustrate performance characteristics for otherwise-similar devices incorporating non-linear layers comprising zinc oxide having thicknesses of 0.16 mm., 0.24 mm. and 0.32 mm. respectively. As the thickness of the separate non-linear layer is increased with respect to the thickness of the phosphor layer, the brightness increases much more rapidly with increasing voltage. This is because an increasing thickness of the layer comprising the non-linear material decreases the capactive reactance across this layer due to the effective increased electrode spacing; i.e., the decreased value of Z increases the potential drop which is realized thereacross at relatively low fields. As the field is increased, however, the electrical resistance across such a non-linear layer decreases rapidly. This in turn decreases in a very rapid fashion the total electrical impedance across such a. layer, with resulting increased potential applied across the phosphor layer. Otherwise expressed, the non-linear zinc oxide layer can be represented as a parallel-connected capacitance and resistance. It is desired to decrease the total impedance in very rapid fashion with increased applied field. By minimizing the capacitive reactance across such a layer, with respect to the capacitive reactance of the separate phosphor layer, any decrease in resistance will be reflected as a much greater decrease in total impedance. it is for this reason that the embodiment 24 as shown in FIG. 2 incorporates an uppermost electrode 22a which is very small in area with respect to the electrode 16 and the floating" electrode 23. With a construction generally shown in FIG. 2, when the area of the electrode 22:! is approximately one one-hundreth of the area of the electrodes 16 and 28, the discrimination ratio of the resulting performance curve can be made as high as 10 to 10 In explanation of the term discrimination ratio, the electroluminescent excitation voltage required to achieve a desired light level (such as five foot lamberts) is determined. This excitation voltage is then halved and the light intensity at this lower exciting voltage is measured. The ratio of the two light intensities is defined as the discrimination ratio. In the usual electroluminescent device, the discrimination ratio is normally about ten. With a construction such as shown in FIG. 1 wherein the bounding electrodes have equivalent areas, the discrimination ratio can be from about thirty (curve B) to 5000 (curve E), depending in part on the relative capacitance of the non-linear layer. Of course the greater the discrimination ratio, the less the crosstall in unwanted areas when the system is used in conjunction with an XY plotter.

While zinc oxide is preferred for best results, other materials can be substituted therefor. As a specific example, it has been found that larger zinc sulfide electroluminescent particlcs have a dielectric constant which increases more rapidly with applied field than smaller zinc sulfide electroluminescent particles. Any of the embodiments as described hereinbefore can utilize as the separate layer of non-linear material, a layer comprising large-particle-diameter electroluminescent phosphor which has been killed for electroluminescent by the addition thereto of selected substance. As a specific example, a killed electroluminescent phosphor can be prepared so as to have an average particle diameter of approximately 20 microns and the electroluminescent phosphor can be prepared so as to have an average particle diameter of five microns. For techniques for preparing materialhaving such particle size, reference is made to copending application SN. 12,616, filedMarch 3, 1960, and owned by the present assignee. The killed electroluminescent phosphor can be prcparedby adding to the raw mix chromium, iron, cobalt or nickel in compound form. As a specific example, 6.01 mole percent of added cobalt nitrate will effectively kill the phosphor for electroluminescent response. In the operation of such a device,

as the field across the spaced electrodes is increased, the dielectric constant of the larger phosphor particles increases at a faster rate than the dielectric constant of the smaller phosphor particles. An increasing dielectric concent phosphor of large average particle size for the nonlinear element. in the case of the zinc oxide layer, the

resistance component of the total impedance decreases as thepotential appliedthereacross increases. In the case of the killed non-linear electroluminescent phosphor of large particle size, the dielectric constant increases at a rate which is faster thanthe rate of increase for the dielectric constant of the smaller-average-particle-size electroluminescent phosphor. In either case, however,

"the effect is the same since the ratio of the potential drop developed across the electroluminescent phosphor divided by the potential drop developed across either of the non- .layer 44 comprising zinc oxide.

linear zinc oxide or killed phosphor layers increases as the applied potential is increased.

In PEG. is shown an electroluminescent XY display system 40 which is capable of presenting images with a high degree of brightness and a high degree of contrast. Briefly this system comprises a layer 42 comprising electroluminescent phosphor, such as described hereinbetore. Adjacent to the phosphor layer is a separate layer 44 comprising field-influenced, non-photoconductive and non-electroluminescent material formed of the non-linear zince oxide as specified hereinbefore. A first electrode means 46, formed as a raster and comprising individual electrode strips, is adjacent the exposed surface of the A second electrode means 48, formed as a raster and comprising individual light-transmitting electrode strips such as tin oxide af- .fixed to a vitreous foundation so is adjacent the exposed surface of the phosphor layer 42. The electrode strips comprising the raster 46 are generally oriented at right angles to the electrode strips comprising the raster 48. A signal generator 52 is connected through a distribution system 53 to the individual electrode strips and comprises means for applying an alternating electric potential of different magnitudes across preselected portions of the first and second electrode means 46 and 48 in accordance with the pattern of the signal to be displayed. Such a general signal generator-distribution arrangement is disclosed in US. Patent No. 2,698,915, dated January 4, 1955.

In FIG. 6 is disclosed an alternative X-Y plotter embodiment 54 wherein the two raster-type electrodes 46 and 43 and the phosphor layer 42 are as described in the system embodiment 40 shown in FIG. 5. This X-Y plotter embodiment 54 is modified, however, by the inclusion of an electrically-insulating foundation as having a' plurality of apertures 53 provided therethrough and aligned to forrna screen-like pattern. The spacing between the individual apertures 53 is selected in accordance with the degree of resolution desired for the display system. The alternating-field-influenced, non-photosenductive and non-electroluminescent material which preferably comprises the zinc oxide as described hereinbefore is used as a filling material 60 for the apertures 53. As a specific example, the foundation 55 can be provided with ten apertures per 30 mnm, each aperture having a diameter of 0.5 mm. The electrode raster 46 of first electrode strips is adjacent the exposed side of the foundation 56 and each strip covers an aligned section of the filled apertures 53 in such manner as to electrically contact that portion of the non-linear material 60 comprising zinc oxide which fills the apertures 53 covered by each electrode strip. Adjacent the opposite side of the foundation 56 are a plurality of electrically-isolated conducting segments s2. which are formed of vacuum-netallized material such as aluminum or silver and each of which segments 52 electrically connect to the electroluminescent phosphor layer 42 and one of the non-linear-material fillings oil in each aperture 58. The basic construction of each element of such a device embodiment 54 is essentially similar to the electroluminescent system which has been illustrated in FIG. 2 and described hereinbefore. As a specific example, the founda ion 56 has a total thickness of approximately one mm. and the layer 42 comprising the phosphor has a thickness of approximately fifty microns. The area. of each of the electrically-isolated segments 62. is approximately four sq. mm. The device embodiment 54 is also connected to a signal genorator-distribution arrangement similar to that illustrated in PEG. 5. it should be realized that the foregoing construction can be modified considerably, such as described hereinbefore for the system embodiments as shown in F163. 1 and 2. In addition, the degree of resolution obtainable can be increased or decreased as desired by increasing or decreasing the number of apertures 58 which are provided through the foundation 56.

The electrically non-linear zinc oxide as described hereinbefore can also be fabricated into an electrically 11011. linear element as as shown in FIG. 7. The parameters governing the design of such an element are the same as set forth for the design of the separate non-linear layer 2%, as shown in 516. 1 and described hereinbefore. As shown, the non-linear layer has electrodes 68 positioned on either side thereof to serve as a means for applying potential thereacross. A foundation 70 provides stability for the element 64.

As an alternative embodiment for the element 64 as shown in FIG. 7, a portion of the zinc oxide can be replaced by finely-divided zinc-sulfide type electroluminescent phosphor which is evenly mixed with the remaining zinc oxide. It has been found that from 5% to 40% by weight of the zinc oxide in such an electrically nonlinear element can be replaced by an equivalent weight of zinc-sulfide type electroluminescent phosphor. The average particle diameter of the mixed phosphor should fall within the range of from" about one-fourth to about four times the average particle diameter of the zinc oxide remaining in the modified non-linear element. Such an embodiment will display electrically non-linear characteristicsand will also electroluminesce with such characteristics that the brightness increases at a greater-than-normal rate with increasing voltage. Apparently where the phosphor and specific zinc oxide are mixed within the proportions as indicated, the lowered impedance of the zinc oxide rather than shorting out the phosphor particles tends to impress a greater field thcreacross. Of course the reduction in the amount or phosphor will decrease the maximum electroluminescent brightness normally obtainable. Likewise, the non-linear characteristics of the element are impaired somewhat when a portion of the zinc oxide is replaced by zinc-sulfide type electroluminestial and which system displays a rapidly increasing brightness with increasing magnitude of alternating potential excitation. Modifications for such a system have been provided and in addition, there has been provided an improved X-Y electroluminescent plotter which has substantially no tendencies for unwanted cross-talk and which plotter can be operated with a very high brightness. There has also been provided a non-linear element and methods for providing specific non-linear material which displays an electrical impedance decreasing the accordance with the magnitude of an electric potential applied thereacross.

While best embodiments of the invention have been illustrated and described in detail, it is to be particularly understood that the invention is not limited thereto or thereby.

I claim:

I. An electroluminescent system displaying a brightness which increases rapidly with increasing alternating potential excitation and comprising: spaced electrodes; means for applying alternating electric potential of different magnitudes across said spaced electrodes; alternating-electric field-infiuenced electroluminescent phosphor means comprising a separate layer included between said spaced electrodes and exhibiting an alternating-electricpotential drop thereacross (V varying with the magnitude of the exciting alternating electric potential applied across said spaced electrodes; insulating means included in sufiicient amount between said spaced electrodes to prevent any direct current flow therebetween and to increase the alternating electric potential required to cause breakdown therebetween; alternating-electricfield-iniiuenced, non-photoconductive and non-electroluminescent means comprising a separate layer included between said spaced electrodes and exhibiting an alternating electric potential drop thereacross (V varying with the magnitude of the alternating electric potential applied across said spaced electrodes; when alternating electric potential is applied across said spaced electrodes, the capacitive reactance displayed by the layer comprising said phosphor means being less than the capacitive reactance displayed by the layer comprising said fieldinl'luenced non-electroluminescent means; and the fieldinfiuenced characteristics of said electroluminescent phosphor means and said field-influenced non-electroluminescent means having such relation to one another that the ratio of potential drops thereacross expressed as V /V increases when the alternating electric potential applied across said spaced electrodes is increased in magnitude.

2. An electroluminescent system displaying a brightness which increases rapidly with increasing alternating potential excitation and comprising: spaced electrodes at least one of which is light transmitting; means for applying alternating electric potential of different magnitudes across said spaced electrodes; alternating-electric field-influenced electroluminescent phosphor means comprising a separate layer included between said spaced electrodes and exhibiting a dielectric constant which increases with the magnitude of the exciting alternating electric potential applied across said spaced electrodes; insulating means included in sufiicient amount between said spaced electrodes to prevent any direct current flow therebetween and to increase the alternating electric potential required to cause breakdown therebetween; and alternating-clectric-field-infiuenced, non-photoconductive and non-electroluminescent means comprising a sepa rate layer included between said spaced electrodes and exhibiting a dielectric constant which increases with the magnitude of the alternating electric potential applied across said spaced electrodes at a rate faster than the rate of increase of the dielectric constant of said electroluminescent phosphor means.

3. An electroluminescent system as specified in claim 2, wherein said electroluminescent phosphor means comprises finely-divided phosphor particles having a predetermined average size, said non-electroluminescent means comprises finely-divided normally electroluminescent phosphor particles having a predetermined average particle size but killed for electroluminescent response by the addition thereto of a predetermined amount of preselected foreign substance, and wherein the average particle size of the particles comprising said field-intluenced non-electroluminescent means is greater than the average particle size of the particles comprising said electroluminescent phosphor means.

4. An electroluminescent system displaying a brightness which increases rapidly with increasing alternating potential excitation and comprising: spaced electrodes at least one of which is light transmitting; means for applying alternating electric potential of different magnitudes across said spaced electrodes; alternating-electricfield-influenced electroluminescent phosphor means comprising a separate layer included between said spaced electrodes and exhibiting a dielectric constant which increases with the magnitude of the exciting alternating electric potential applied across said spaced electrodes; insulating means included in sufiicient amount between said spaced electrodes to prevent any direct current flow therebetween and to increase the alternating electric potential required to cause breakdown therebetween; and alternating-electric-field-infiuenced, non-photoconductive and non-electroluminescent means comprising a separate layer included between said spaced electrodes and exhibiting an impedance which decreases with increasing magnitude of the alternating electric potential applied across said spaced electrodes.

5. An electroluminescent system as specified in claim 4, wherein said field-influenced non-electroluminescent means comprises, a layer having a predetermined thickness and comprising a mixture of from 15% to by weight dielectric material and from to 20% by weight finely-divided zinc oxide, and the finely-divided zinc oxide in such mixture having an average particle diameter of from about one-fourth to about one onehundredth of the predetermined thickness of the layer comprising said field-influenced non-electroluminescent means.

6. An electroluminescent system displaying a brightness which increases rapidly with increasing alternating potential excitation, said system comprising: a first electrode; a light-transmitting second electrode spaced from said first electrode; means for applying an alternating electric potential of different magnitudes across said first and second electrodes; a third electrode positioned between and spaced from said first and said second electrodes; the area of said first electrode being considerably smaller than the area of said second and third electrodes; alternating; electric-field influenced electroluminescent phosphor means comprising a separate layer included between said second and third electrodes and exhibiting an alternating electric potential drop thereacross (V varying with the magnitude of the exciting alternating electric potential applied across said first and second electrodes; insulating means included in sufiicient amount between said second and third electrodes to prevent any direct current flow therebetween and to increase the alternating electric potential required to cause breakdown therebetween; alternating electric-field influenced, nonphotoconductive and non-electroluminescent means comprising a separate layer included between said first and second electrodes and exhibiting an alternating electric potential drop thereacross (V varying with the magnitude of the alternating electric potential applied across said first and second electrodes; and the field-influenced characteristics of said electroluminescent phosphor means and said non-electroluminescent means having such relation to one another that the ratio of potential drops thereacross expressed as V /V increases when the alternating electric potential applied across said first and secondvelectrodes is increased.

"11 7. An electroluminescent system as specified in claim 6, wherein said fi lll-ll'lflll6I1C6d non-electroluminescent means comprises, a layer having a predetermined thickness and comprising a mixture of from 15% to 80% by weight dielectric material and from 85% to by weight finelydivided zinc oxide, and the finely-divided zince oxide in such mixture havin an average particle diameter of from about one-fourth to about one onehundredth or" the predetermined thickness of the layer comprising said field-influenced non-electroluminescent means.

8. An electroluminescent display system for presenting images with a high degree of brightness and a high degree or" contrast and comprising: a layer comprising electroluminescent phosphor; a layer thereover having a predetermined thickness greater than the thickness of said layer comprising electroluminescent phosphor and formed of field-infiucnced, non-photoluminescent and non-electroluminescent material comprising a mixture of from 15 to 80% by weight dielectric material and from 85 to 20% by weight finely-divided zinc oxide; the finelydivided zinc oxide in said layer comprising field-influ- 'enced non-electroluminescent material having an average particle diameter of from about one-fourth'to about one one-hundredth of the predetermined thickness of such layer; a first electrode means formed as a raster of individual electrode strips and adjacent to the exposed surface of said layer comprising fieldinfluenced nonelectroluminescent material; second electrode means formed as a raster of individual light-transmitting electrode strips and adjacent to the exposed surface of said layer comprising electroluminescent phosphor and generally oriented at right angles to the raster formed by said first electrode means; insulating means included in sufficient amount between said first and second electrode means to prevent any direct current fiow therebetween and to increase the alternating electric potential required to cause breakdown therebetween; and means for applying an alternating electric potential of different magnitudes across preselected portions of said first and second electrode means in accordance with the pattern of the signal desiredto be displayed.

9. An electroluminescent display system for presenting images with a high degree of brightness and a high degree of contrast and comprising: an electrically-insulating foundation; a plurality of apertures provided through said foundation and aligned to form a screen-lilac pattern with the spacing between apertures selected in accordance with the resolution desired for said display system; alternating-electric-field-infiuenced, non-photoconductive and non-electroluminescent means comprising a filling material for said apertures; a first electrode means formed as a raster of individual electrode strips and adjacent to one side of said foundation with each of such strips covering a different single line of said apertures and electrically contacting that portion of said non-electroluminescent means which fills the apertures so covered; alternating electric-field influenced electroluminescent phosphor means comprising a layer affixed to the side of said foundation opposite said first electrode means and electrically connecting to said non-electroluminescent means filling said apertures; the layer comprising said electroluminescent phosphor means having a thickness less than the thickness of said foundation; second electrode means formed as a raster of individual light-transmitting electrode strips over the layer comprising said electroluminescent phosphor means and generally oriented at right angles to the raster formed by said first electrode means; means for applying an alternating electric potential of difierent magnitudes across preselected portions of said first and second electrode means in accordance with the pattern of the signal desired to be dis played; insulating means included in sufiicient amount between said first and second electrode means to prevent second electrode means; and the field-influenced characteristics of said electroluminescent phosphor means and said field-influenced non-electroluminescent means having such relation to one another-that the ratio of potential drops thereacross expressed as f /V increases when the alternating electric potential applied across said first and second electrode means is increased.

10. A display system as specified in claim 9, wherein an additional electrode means formed as a plurality of electrically-isolated segments is positioned between said electroluminescent phosphor means and said foundation, each of the segments comprising said additional electrode means having an area greater than the cross-sectional area of said apertures, and each of the segments forming said additional electrode means electrically connecting said electroluminescent phosphor means and a different portion of said field-influenced non-electroluminescent means as fills one of said apertures.

11. The method of preparing non-linear zinc oxide which displays an electrical impedance which decreases in accordance with the magnitude of an electric field applied thereacross, which method comprises; mixing the following in the following proportions: 1 mole zinc oxide, from about 0.01 gram atom percent to 1 gram atom percent of at least one of the group consisting of copper, lithium and silver in compound form, and from about 0.01 gram atom percent to about 1 gram atom percent of at least one of the group consisting of aluminum, scandium, gallium and indium in compound form, with the total gram atoms of said first group in said mixture constituting from about one-half to two times the total gram atoms of said second group in said mixture; and firing said mixture at a temperature of from about 1000" C. to about 1300 C. in an oxygen-containing atmosphere for a period oi from about ten minutes to about sixhours.

12. The method of preparing non-linear zinc oxide which displays an electrical impedance which decreases in accordance with the magnitude of an electric field applied thereacross, which method comprises: mixing the following in the following proportions: 1 mole zinc oxide, from about 0.01 gram atom percent to 1 gram atom percent of copper as sulphate and from about 0.01 gram atom percent to about 1 gram atom percent of aluminum as sulphate with the total gram atoms of copper in said mixture constituting from about one-half to about two times the total gram atoms of aluminum in said mixture; and firing said mixture at a temperature of from about 1100 C. to about 1200 C. in an air atmosphere for a period of about one hour.

13. The method of preparing non-linear zinc oxide which displays an electrical impedance which decreases in accordance with the magnitude of an electric field applied thereacross, which method comprises, firing a mixture of finely-divided zinc oxide and from 0.1% to 30% by Weight of finelyivided sodium chloride in an oxygen-containing atmosphere at a temperature of from 800" C. to 1200 C. for a period of from about 10 minutes to about six hours, and thereafter separating residual sodium chloride from the fired mixture.

14. The method of preparing non-linear zinc oxide which displays an electrical impedance which decreases in accordance with the magnitude of an electric field applied thereacross, which method comprises, firing a 13 mixture of finely-divided zinc oxide and about by weight of finely-divided sodium chloride in an oxygen atmosphere at a temperature of about 1000 C. for about one hour, and thereafter water Washing the fired mixture to remove residual sodium chloride therefrom.

15. The method of preparing non-linear Zinc oxide which displays an electrical impedance which decreases in accordance with the magnitude of an electric field applied thereacross, which method comprises, firing zinc carbonate in a partially-closed container which will permit egress of pressures therefrom at a temperature of from about 1000 C. to about 1200" C. for a period of from 10 minutes to about six hours.

16. A non-linear element displaying an electrical impedance which decreases in accordance with the magnitude of an electric potential applied thereto, said element comprising, electrodes separated by a predetermined spacing and adapted to have an electric potential applied therebetween; material sandwiched between said spaced electrodes and comprising a mixture of from 15% to 80% by weight dielectric material and from 85% to by Weight finely-divided zinc oxide, and the finelydivided zinc oxide in said mixture having an average particle size of from about one-fourth to about one one-hundredth of the predetermined spacing between said electrodes.

17. A non-linear element displaying an electrical impedance which decreases in accordance with the magnitude of an electric potential applied thereto and comprising, a material layer having a predetermined thickness, said layer comprising a mixture of from 30% to 40% by Weight dielectric material and from 70% to 60% by Weight finely-divided Zinc oxide, the finely-divided zinc oxide in said mixture having an average particle diameter of from about one-fourth to about one one-hundredth of the predetermined thickness of said layer, and means for applying an electric potential across said layer.

18. An electrically non-linear element as specified in claim 17, wherein from 5% to by weight of the zinc oxide in such element is replaced by an equivalent Weight of zinc-sulfide type electroluminescent phosphor evenly mixed throughout said element, and the average particle diameter of said mixed phosphor falling within the range of from about one-fourth to about four times the average particle diameter of remaining zinc oxide.

19. A non-linear element displaying an electrical impedance which decreases in accordance with the magnitude of an electric potential applied thereto and comprising, a material layer having a predetermined thickness, said layer comprising a mixture of from 30% to 40% by Weight dielectric material and from 70% to 60% by Weight finely-divided zinc oxide, the finely-divided zinc oxide in said mixture having an average particle diam eter of about one-tenth of the predetermined thickness of said layer, and means for applying an electric potential across said layer.

20. A non-linear element displaying an electrical impedance which decreases in accordance with the magnitude of an electric potential applied thereto and comprising, a material layer having a predetermined thickness of about microns, said layer comprising a mixture of from 30% to 40% by Weight dielectric material and from 70% to by weight finely-divided zinc oxide, the finely-divided zinc oxide in said mixture having an average particle diameter of from about one-tenth of the predetermined thickness of said layer, and means for applying an electric potential across said layer.

References Cited in the file 01. this patent UNITED STATES PATENTS 1,047,540 Lederer Dec. 17, 1912 1,208,629 Oberlander Dec. 12, 1916 2,866,922 Matarese Dec. 30, 1958 2,942,150 Ullery June 21, 1960 

1. AN ELECTROLUMINESCENT SYSTEM DISPLAYING A BRIGHTNESS WHICH INCREASES RAPIDLY WITH INCREASNG ALTERNATING POTENTIAL EXCITATION AND COMPRISING: SPACED ELECTRODES; MEANS FOR APPLYING ALTERNATING ELECTRIC POTENTIAL OF DIFFERENT MAGNITUDES ACROSS SAID SPACED ELECTRODES; ALTERNATING-ELECTRIC-FIELD-INFLUENCED ELECTROLUMINESCENT PHOSPHOR MEANS COMPRISING A SEPARATE LAYER INCLUDED BETWEEN SAID SPACED ELECTRODES AND EXHIBITING AN ALTERNATING-ELECTRICPOTENTIAL DROP THEREACROSS (V1) VARYING WITH THE MAGNITUDE OF THE EXCITING ALTERNATING ELECTRIC POTENTIAL APPLIED ACROSS SAID SPACED ELECTRODES; INSULATING MEANS INCLUDED IN SUFFICIENT AMOUNT BETWEEN SAID SPACED ELECTRODES TO PREVENT ANY DIRECT CURRENT FLOW THEREBETWEEN AND TO INCREASE THE ALTERNATING ELECTRIC POTENTIAL REQUIRED TO CAUSE BREAKDOWN THEREBETWEEN; ALTERNATING-ELECTRICFIELD-INFLUENCED, NON-PHOTOCONDUCTIVE- AND NON-ELECTROLUNINESCENT MEANS COMPRISING A SEPARATE LAYER INCLUDED BETWEEN SAID SPACED ELECTRODES AND EXHIBITING AN ALTERNATING ELECTRIC POTENTIAL DROP THEREACROSS (V2) VARYING WITH THE MAGNITUDE OF THE ALTERNATING ELECTRIC POETNTIAL APPLIED ACROSS SAID SPACED ELECTRODES; WHEN ALTERNATING ELECTRIC POTENTIAL IS APPLIED ACROSS SAID SPACED ELECTRODES, THE CAPACITIVE REACTANCE DISPLAYED BY THE LAYER COMPRISING SAID PHOSPHOR MEANS BEING LESS THAN THE CAPACTIVE REACTANCE DISPLAYED BY THE LAYER COMPRISING SAID FIELDINFLUENCED NON-ELECTROLUMINESCENT MEANS; AND THE FIELDINFLUENCED CHARACTERISTICS OF SAID ELECTROLUMINESCENT PHOSPHOR MEANS AND SAID FIELD-INFLUENCED NON-ELECTROLUMINESCENT MEANS HAVING SUCH RELATION TO ONE ANOTHER THAT THE RATIO OF POTENTIAL DROPS THEREACROSS EXPRESSED AS V1/V2 INCREASES WHEN THE ALTERNATING ELECTRIC POTENTIAL APPLIED ACROSS SAID SPACED ELECTRODES IS INCREASED IN MAGNITUDE. 